I. Introduction

Solitary waves, interpreted as negative potential pulses traveling up the magnetic field, were first observed in the auroral zone [Temerin et al., 1982, Bostromet al., 1988]. Solitary structures, interpreted as electron holes, were identified in the distant plasma sheet [Matsumoto et al., 1994]. Estimates of the velocity of solitary waves in the auroral zone were made by several groups, using both electric field and Langmuir probe instruments. The solitary waves have been associated with the ion acoustic mode and have been identified as ion holes. The first observations, made by the S3-3 satellite electric field instrument, indicated that the waves must be traveling faster than ~50 km/s [Temerin et al., 1982]. Viking satellite Langmuir probe observations [Bostrom et al, 1988] indicated that the speed was slower (~5-50 km/s). Recent Polar observations [Mozer et al., 1997; Bounds et al., 1998] have shown that the propagation speed is usually 100s of km/s. McFadden [1998] has provided a possible explanation for the discrepancy between the Viking observations and those from S3-3 and Polar based on the operation of Langmuir probes in a low density plasma. In addition, utilizing data from the FAST spacecraft, Ergun et al. [1998] have identified a new type of solitary wave, associated with cold electron beams, which propagates upward at ~4500 km/s. These waves have an electromagnetic signature and are interpreted as electron holes traveling with the beam. Similar structures in the low altitude Polar data were described by Mozer et al. [1997]. Preliminary results from Polar have also provided evidence for fast moving solitary waves at high altitudes [Cattell et al., 1998a; Franz et al, 1998].

In this letter, data from Polar are used to determine the propagation speeds of solitary waves observed in the plasma sheet boundary and the high altitude cusp. We compare the observed solitary wave characteristics to plasma properties in order to address the similarities and differences between the solitary waves observed in these regions and the solitary waves previously identified in low altitude auroral zone data. In Section II, the data sets and methodology are described. Solitary wave examples from both regions are presented in Section III. The significance of the propagation speed differences will be discussed in Section IV, using theories of both ion acoustic and electron acoustic solitary waves.